For a network component accessing an end point of a network or subnetwork, availability and capacity of a network connection to another end point in the network are important characteristics of the network connection, independent of a physical routing path used in the network for providing the network connection. Availability refers to the degree to which the network connection is in a specified state of operation, such as the existence of network connectivity with a specified capacity. Capacity refers to the bit rate provided by the network connection. Adaptive Modulation (AM), for example in a link using microwave equipment, is an efficient tool to increase the link capacity. However, the availability of the increased link capacity is often reduced.
For example, a microwave link is planned for a small Bit Error Rate (BER) at a low modulation level during major part of its operation time, resulting in approximately 99.999% availability of the link. Adaptive modulation means that in good radio conditions, the microwave channel is able to provide significantly higher capacity with lower availability of approximately 99.995% to 99.9% by using increased modulation levels. The used modulation level thus depends on current radio channel conditions. Automatic switching between the modulation levels provides the highest available link capacity for the current radio channel conditions.
The advantage of adaptive modulation can be utilized efficiently in networks carrying both services with “strict guarantees”, which services typically need moderate but guaranteed bandwidth with high availability, and “best effort” services, which can utilize significantly higher link capacity but can tolerate service degradation. Examples for services with strict guarantees include services with Guaranteed Bit Rate (GBR), voice services and video call services. The availability required by services with strict guarantees can be 99.999%. Data communication services are often best effort services.
If a double-connected network is deployed, e.g., using microwave links, it is possible to protect the services carried by the lowest modulation level when the link capacity is going down to zero by using a protection switching mechanism. The protected traffic is rerouted to an alternative path, which does not contain the failed link. Herein, “failed” and “fault” refer to the outage of network connectivity.
Typical services, for example in the context of telecommunication, can tolerate only a very short time of outage and/or capacity degradation. Consequently, a restoration time is limited, for example to 50 ms. Achieving the short restoration time implies an even more rapid fault detection.
Conventional techniques for achieving the short restoration time use the fault detection mechanism described in the Recommendation ITU-T Y.1731 (“OAM Functions and Mechanisms for Ethernet based Networks”) of the International Telecommunication Union as published in July 2011 and the standard document IEEE 802.1ag (“IEEE Standard for Local and Metropolitan Area Networks”) for Connectivity Fault Management (CFM) as part of the Ethernet OAM Architecture for Operations, Administration and Maintenance (OAM).
To detect a fault between Maintenance End Points (MEPs), continuity check messages are periodically exchanged between them, which messages are referred to as Continuity Check Messages (CCMs) in the Recommendation ITU-T G.8031/Y.1342 or Continuity Check (CC) messages in networks using Multiprotocol Label Switching (MPLS). The fault is indicated to the MEP by the loss of one or more continuity check messages.
Conventional techniques using continuity check messages, including the Ethernet OAM mechanism and the MPLS mechanism, can only handle the case when the capacity of a link, e.g., a microwave link, is going down to zero. A microwave link capacity degradation, which can violate a guaranteed bit rate or a minimum bit rate of a service, is not detected. For example, the volume of GBR services can be higher than the capacity provided by the lowest modulation level of adaptive modulation. In this case, the service is degraded or collapses, when the adaptive modulation switches to a certain modulation level, since the required bandwidth is not available anymore. However, the fault-handling mechanism is not activated, because the link capacity is not zero and the continuity check messages are not lost.
In the context of the Ethernet OAM mechanism, it has been proposed to additionally use the number of dropped CCMs as an indicator of a degree of degradation. The number of lost CCMs, however, depends on other parameters, such as the number of links between the MEPs and a priority level of the CCMs. Thus, the number of lost CCMs is an unreliable indicator for capacity degradation.
A further solution proposed in the context of the Ethernet OAM mechanism extends the standard Ethernet OAM architecture by using a notification message in addition to the CCMs. However, sending and processing the notification message requires amendments beyond the standard architecture at multiple places throughout the network and causes additional signaling through the network. Furthermore, due to the additional time for transmitting and interpreting the notification message, the restoration time does not meet certain Quality of Service (QoS) requirements.